CALCULATED POTENTIAL SURFACES FOR THE DESCRIPTION OF THE EMISSION-SPECTRUM OF THE C2H RADICAL

Ab initio MRD-CI calculations are reported for the CC stretch and CCH bending potential surfaces for the lowest-lying electronic states of the ethynyl radical. It is found that the energies of the fully relaxed π → π* states of this system are as much as 2.3 eV below the corresponding values at the 2Σ+ ground state (linear) equilibrium structure. The fact that the energy minima for the C2H excited states occur for widely different internuclear arrangements than that of the ground state is shown to be very consistent with the experimental observation that the electronic spectrum of C2H has a much different appearance in emission than it does in absorption studies. On the basis of both calculated energy differences and oscillator strength results it is found that the most likely assignment for the broad emission feature in the 4000-6000 Å region of the C2H spectrum is caused by vertical excitation from the lowest vibrational levels of the 32A′ (and also 22A″) upper states to the 22A′, 2A′ and 2A″ species which correlate with X2Σ+ and A2Π for linear nuclear arrangements; the possible mechanisms for production of 32A′ and 22A″ upon photolysis of acetylene are also discussed. The possibility that spinforbidden transitions are involved at longer wavelengths in this spectral region is also considered. Finally it is pointed out that a potential crossing of the linear X ̃2Σ+ and A2Π species calculated to occur in the neighborhood of RCC = 2.6 bohr, which becomes sharply avoided upon molecular bending, would be expected to lead to the type of irregular vibronic structure in the 3600 cm-1 region of the C2H IR spectrum recently observed by Jacox. © 1979.